The present invention relates to a power transmission apparatus for a hybrid vehicle and a method for controlling the apparatus which has two driving means of an engine and a motor, and particularly to those enabling high transmission efficiency and a reduction in the shock at shifting.
As a power transmission apparatus for a hybrid vehicle, which has both engine and a motor as vehicle driving means, the following are disclosed.
First, Japanese Laid-Open Patents 8-183347 and 8-183348 disclose the power transmission apparatus, in which an engine output shaft is connected to the carrier of a planetary gear and a generator input shaft to the sun gear, thereby transmitting the combined output of a ring gear and a motor to driving wheels. This power transmission apparatus for a hybrid vehicle distributes the power of an engine and the motor by a planetary gear unit so that the motor may bear the larger load than the engine while driving at a high speed and power.
Secondly, a power transmission apparatus for a hybrid vehicle, disclosed in Japanese Laid-Open Patent 9-277847, in which an engine output shaft is connected to the ring gear of a planetary gear unit and a motor generator output shaft to the sun gear, thereby transmitting the rotation of the carrier to driving wheels through an automatic transmission.
Of the power transmission apparatus for a hybrid vehicle mentioned above, the former apparatus has suffered from the problem that a large amount of power is required for the motor since it has no transmission between the motor and the driving wheels.
On the other hand, the latter apparatus is free of the above-mentioned problem since a transmission is provided between the carrier, to which the combined output of the engine and the motor is transmitted, and the driving wheels. When a shifting from the first to the second shifting level is performed by the transmission, it is not possible to shift smoothly unless the number of revolutions of the driving wheel side shaft and that of the second shifting level shaft are synchronized with the number of revolutions of the engaging gears. It will be one of the causes for the shifting shock. The synchronization will require an additional synchronizing mechanism, which results in the complex apparatus. If a transmission with a torque converter or a continuously variable transmission with a belt is employed, it will possibly raise a problem of power transmission loss due to the slip of the torque converter or the belt.
The present invention to assess the problems described above provides a power transmission apparatus for a hybrid vehicle and a method for controlling the apparatus, which enable the reduction in the shifting shock as well as high power transmission efficiency.
The present invention provides a power transmission apparatus for a hybrid vehicle including two driving means; an engine and a motor generator which has a rotor to rotate supplied with electricity and serves as a power generator supplied with torque, comprising; a planetary gear unit including at least a sun gear, a carrier and a ring gear; and a counter shaft for transmitting power to driving wheels of the vehicle, wherein the rotor of the motor generator is connected to the sun gear, an engine output shaft and a first main shaft which is capable of transmitting torque to the counter shaft through first gears in a first plurality of transmission gear ratios are connected to the carrier, and torque of the ring gear can be transmitted through second gears to a second main shaft which is capable of transmitting torque to the counter shaft through third gears in a second plurality of transmission gear ratios.
The power transmission apparatus for a hybrid vehicle transmits the combined power of the engine and the motor generator, distributing it to a first main shaft and a second main shaft, thereby allowing both of them to transmit torque to a counter shaft through gears in multiple shift ratios. When a shifting is performed from the first shift level, in which the torque is transmitted to the counter shaft with one of the first and the second main shafts, to the second shift level, in which the torque is transmitted to the counter shaft with the other remaining main shaft, it is possible to keep one main shaft driving and control the number of revolutions of the other main shaft simultaneously, thereby synchronizing the number of revolutions of the other shaft with that of the counter shaft to engage them smoothly. Provision of the two power transmission paths allows one of them to transmit the power to the counter shaft and the other to be synchronized with it in parallel, thereby achieving a small shifting shock and an excellent acceleration by obviating the shifting time lag due to the disconnection of the power.
The present invention according to claim 2 provides the power transmission apparatus for a hybrid vehicle of claim 1, wherein the engine output shaft and the carrier are connected through a clutch, which is capable of connecting and disconnecting power selectively.
The introduction of a clutch, provided between the engine output shaft and the carrier, which is capable of connecting and disconnecting the power selectively, permits an energy efficient power transmission by obviating the energy loss due to the engine rotation, disengaging the clutch and stopping the engine, when a hybrid vehicle is driven by a motor generator only. The engagement and disengagement of gears will also be performed smoothly by disengaging the clutch at the engine start.
The present invention according to claim 3 or 4 provides a method for controlling the power transmission apparatus for a hybrid vehicle of claim 1 or 2 respectively, the method comprising the steps of; when a shifting is performed from a first shifting level to a second shifting level, synchronizing one of the first and second main shafts with the counter shaft in the second shifting level by controlling a number of revolutions of the motor generator, while the other of the shafts engages with the counter shaft in the first shifting level; and performing a transition from the first shifting level to the second shifting level by connecting one of the shafts with the counter shaft while disconnecting the other of the shafts from the counter shaft.
Controlling the power transmission apparatus for a hybrid vehicle according to claim 1 or claim 2 this way allows the shifting from the first to the second shift level smoothly, since the connection is performed after the completion of the rotational synchronization by the motor generator, when a shaft is connected to the counter shaft in the second shift level.
The first and the second shift levels are defined as a shift level and another shift level different from the former, not meaning a low speed (first speed) or second speed. A shift from the first to the second shift level includes shifting from the second to the third speed or from the third to the low speed (first speed).
The connection according to claim 1 and claim 2 is defined as the conditions in which the torque is transmitted irrespective of the existence of a power transmission means such as a gear in between.